1. Field of the Invention
The present invention is related to a method of reporting a measurement report event, and more particularly, to a method of reporting a measurement report event with low call drop rate.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third Generation Partnership Project (3GPP), provides a high data rate, low latency, packet optimization, and improved system capacity and coverage using radio access technology (RAT). In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) is configured to communicate with a user equipment (UE) and a core network (CN). A core network may be a network device such as a mobility management entity (MME) or a serving gateway.
Mobility management in the LTE system mainly includes cell selection/reselection, handover and related measurements. Handover refers to the transfer process of management from a current serving cell to a target cell and is performed in all cellular mobile communication systems. There are two types of handover: hard handover and soft handover. Hard handover may be performed in all non-CDMA (code division multiple access) systems. For example, in a time division multiple access (TDMA) system, the current connection is broken before making a new connection to the target cell. Such break-before-make hard handover only requires simple hardware structure, but suffers from high call drop rate. Soft handover may be performed in all CDMA systems which adopt orthogonal codes and rake receivers. Such make-before-break soft handover can reduce call drop rate during handover, but requires more complicated hardware structure or algorithm.
The cells around a UE can be categorized in 3 different sets: active set, candidate set and monitored set. A cell of the active set is configured to establish connection with a mobile device during handover. A cell of the candidate set does not establish connection with a mobile device during handover, but its signal quality is qualified to be included in the active set. A cell of the monitor set does not have signal quality qualified to be included in the active set. The UE is configured to measure the signals from all cells within its communication range and perform handover accordingly. Soft handover includes three major modes: radio link addition, radio link removal, and radio link replacement. The decision of whether a cell belongs to the active set or the monitor set is made by the UE. According to corresponding trigger conditions, the UE may add an unconnected cell into the active set or remove a connecting cell from the active set.
The network is configured to send measurement criteria to UE via broadcast control channel (BCCH) for managing handover, radio bearer control (RBC) and positioning. The UE is then configured to evaluate whether the signals of the cells in the active set and the monitor set satisfy trigger conditions according the parameters provided by the network. The radio network controller (RNC) is notified when a trigger condition is satisfied for determining whether a corresponding handover should be performed. The parameters provided by the network may include reporting range (RR), hysteresis (H), and weighting parameter (W). These parameters are qualitatively defined in 3GPP specifications, but quantitative calculation or adjustment is performed and dynamically calibrated by the network according the geological environment and loading condition of the cells during setup stage.
For example, the trigger condition of measurement report event 1B associated with radio link removal defined in 3GPP specifications is depicted as follows:
                                                        10              ·              Log                        ⁢                                                  ⁢                          M              OLD                                +                      CIO            OLD                          ≤                              W            ·            10            ·                          Log              (                              1                /                                                      ∑                                          i                      =                      1                                                              N                      A                                                        ⁢                                      (                                          1                      /                                              M                        i                                                              )                                                              )                                +                                                    (                                  1                  -                  W                                )                            ·              10              ·              Log                        ⁢                                                  ⁢                          M              BEST                                -                      (                                          R                                  1                  ⁢                  b                                            +                                                H                                      1                    ⁢                    b                                                  /                2                                      )                                              (        1        )            
Mi˜MNA ′ MBEST and MOLD are primary common pilot channels (CPICHs) of all cells within the communication range, a cell having the best signal quality and a current cell, respectively. Other parameters are clearly defined in 3GPP specifications.
However, the UE may trigger the measurement report event 1B inadequately, thereby causing unnecessary handover. For example, if the parameters are set for a specific environment as W=0, H1b=0, R1b=4(dB) and CIOOLD=−4(dB), which are applied in equation (1). The resulting equation (2) is depicted as follows:10·LogMOLD−4≦0+(1−0)·10·LogMBEST−4  (2)
Therefore, the trigger condition of the measurement report event 1B in the specific environment as illustrated by equation (2) is always satisfied regardless of the values of MBEST and MOLD. In other works, the cell having the best signal quality may be removed from the active set when the measurement report event 1B is adequately triggered, thereby increasing call drop rate.